The invention generally relates to magnetic resonance imaging (MRI), and, more particularly, to cryogenically cooled radiofrequency (RF) coils and RF coil arrays for use in MRI.
Generally, MRI is a well-known imaging technique. A conventional MRI device establishes a homogenous magnetic field, for example, along an axis of a person's body that is to undergo MRI. This homogeneous magnetic field conditions the interior of the person's body for imaging by aligning the nuclear spins of nuclei (in atoms and molecules forming the body tissue) along the axis of the magnetic field. If the orientation of the nuclear spin is perturbed out of alignment with the magnetic field, the nuclei attempt to realign their nuclear spins with an axis of the magnetic field. Perturbation of the orientation of nuclear spins may be caused by application of radio frequency (RF) pulses. During the realignment process, the nuclei precess about the axis of the magnetic field and emit electromagnetic signals that may be detected by one or more surface coils placed on or about the person.
Imaging time is largely a factor of desired signal-to-noise ratio (SNR) and the speed with which the MRI device can fill the k-space matrix. In conventional MRI, the k-space matrix is filled one line at a time. Although many improvements have been made in this general area, the speed with which the k-space matrix may be filled is limited. To overcome these inherent limits, several techniques have been developed to simultaneously acquire multiple lines of data for each application of a magnetic field gradient. These techniques, which may collectively be characterized as “parallel imaging techniques”, use spatial information from arrays of RF detector coils to substitute for the encoding which would otherwise have to be obtained in a sequential fashion using field gradients and RF pulses. Two such parallel imaging techniques that have recently been developed and applied to in vivo imaging are SENSE (SENSitivity Encoding) and SMASH (simultaneous acquisition of spatial harmonics). Both techniques include the parallel use of a plurality of separate receiving elements, with each element having a different respective sensitivity profile, and combination of the respective spin resonance signals detected enables a reduction of the acquisition time required for an image (in comparison with conventional Fourier image reconstruction) by a factor which in the most favorable case equals the number of the receiving members used.
The effectiveness of parallel imaging depends on signal-to-noise ratio (SNR), homogeneity of magnetic field and the field-of-view (FOV) coverage. A particular drawback to many parallel imaging techniques results when the component coil sensitivities of the RF coil array are either insufficiently well characterized or insufficiently distinct from one another. These instabilities may manifest as localized artifacts in the reconstructed image, or may result in degraded SNR. SNR is defined as the ratio of signal strength of the image and background noise.
More recently, parallel imaging techniques have been further developed to exploit multiple receive channels, for example 8, 16 or 32 channels receiving signals from 8, 16 or 32 receiver coils respectively. In a typical multiple coil array arrangement, several adjacent coils are provided for receiving signals during imaging. However, there are a number of design challenges in providing the capability of multiple receive channels and multiple coils. For example, the size of coils needed to support a 32-channel MRI system must be sufficiently small to fit within a typical 40 cm field of view of a conventional MRI system, or a smaller field of view for some applications. Additionally, the coil size and corresponding arrangement within a coil array will present with inherent inductive coupling and signal-to-noise ratio (SNR) issues which both can negatively impact the quality factor (Q) and loading factor of the coils and overall performance of the coils and MRI system during imaging.
The loading factor is the ratio of unloaded Q to loaded Q (when the coil is loaded by being placed on the subject), where the quality factor Q is a measure of the coil resonance frequency divided by the width of the coil resonance. The loading factor serves as a measure of the ratio of total resistive losses arising from the coil and the imaging subject divided by the losses from the coil alone. High loading factors mean most of the noise is coming from the subject, not the coil. Therefore, the need to improve SNR becomes more important for parallel imaging applications.
Typically, an RF coil array achieves higher SNR if placed closer to the part of subject being imaged. It has been found that cooling the RF coil, such as by immersion of the RF coil in liquid cryogens such as liquid nitrogen or liquid helium or alternatively, immersing RF coils in liquid nitrogen dewars made of PVC, foam, plastic or glass, will also improve the signal-to-noise ratio by reducing resistive losses in the coil but requires careful handling because the cooled RF coils may come into close contact with the subject being imaged. Accordingly, it is desirable to implement cooled RF coils and/or coil arrays in MRI systems that (among other aspects) provide increased SNR and patient safety, particulary for use of parallel imaging technique.
Practical cryogenic cooled RF coil arrays and/or RF surface coils are also challenging to build. The relatively small space available between the coil and the patient required to obtain high quality images limits the type of insulation to a vacuum structre. The cryogen must be carefully contained inside this vacuum vessel or structure in a hermetically sealed tube or chamber in order to be thermally efficient. In most cryogenic systems, a metal tube is employeed, but this is problematic for MRI systems because the metal will interfere with the RF field of the MRI system. Similar problems exist for cryogenically cooling single RF coils employed as RF antennas in a MRI system.
Therefore, what is needed is a RF coil assembly adapted for use in a multi-channel or parallel imaging MRI system that overcomes the challenges described above.